Wide zero field interaction distributions in the high spin EPR of metalloproteins
Résumé
The EPR spin Hamiltonian parameters of transition ion active centres in frozen aqueous solutions of proteins are distributed as a reflection of distributions in spacial conformations. This phenomenon is generally referred to with the generic term g-strain, however, its manifestation is not limited to a distribution in the g-values. The equivalent name D-strain applies to the situation common for biological half-integer high-spin systems whose powder EPR shape is predominantly modified through the distributive effects in the second order terms of the spin-spin interaction Hamiltonian. A simple, one-parameter model is developed to account for D-strain, and this forms the basis for an efficient and practical numerical analysis procedure for D-strained spectra in the weak field limit. Analysis of S = 5/2, 7/2, and 9/2 protein examples is used to illustrate the drastic modification of relative intensities and widths and the occurrence of extra turning points in these spectra as a consequence of D-strain
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